Chemokines are a group of small (approximately 8 to 14 kD), mostly basic, structurally related molecules that regulate cell trafficking of various types of leukocytes through interactions with a subset of 7-transmembrane, G protein-coupled receptors. Chemokines play fundamental roles in the development, homeostasis and function of the immune system, and have effects on cells of the central nervous system as well as on endothelial cells that are involved in angiogenesis or angiostasis. Chemokines are divided into 2 major subfamilies, CXC and CC, based on the arrangement of the first 2 of the 4 conserved cysteine residues which occur in chemokine protein sequences; the 2 cysteines are separated by a single amino acid in CXC chemokines and are adjacent in CC chemokines. CXC chemokines are further subdivided into ELR and non-ELR types based on the presence or absence of a glu-leu-arg sequence (ELR motif) adjacent and N-terminal to the CXC motif. A new classification system which groups the different chemokines was presented by Zlotnik and Yoshie (2000, Immunity 12, 121-127) and is presented in Table 1.
TABLE 1classification of CXC chemokines and chemokine receptors(From Zlotnik and Yoshie cited above).CXC Chemokine FamilySystematicHumanHumanChemokineNameChromosomeLigandMouse LigandReceptor(s)CXCL14q12-q13GROalpha/MGSA-alphaGRO/KC?CXCR2 > CXCR1CXCL24q12-q13GRObeta/MGSA-betaGRO/KC?CXCR2CXCL34q12-q13GROgamma/GRO/KC?CXCR2MGSA-gammaCXCL44q12-q13PF4PF4UnknownCXCL54q12-q13ENA-78LIX?CXCR2CXCL64q12-q13GCP-2CKa-3CXCR1, CXCR2CXCL74q12-q13NAP-2UnknownCXCR2CXCL84q12-q13IL-8UnknownCXCR1, CXCR2CXCL94q21.21MigMigCXCR3CXCL104q21.21IP-10IP-10CXCR3CXCL114q21.21I-TACUnknownCXCR3CXCL1210q11.1SDF-SDF-1CXCR41alpha/betaCXCL134q21BLC/BCA-1BLC/BCA-1CXCR5CXCL14UnknownBRAK/bolekineBRAKUnknownCXCL15UnknownUnknownLungkineUnknown
The human chemokine GCP 2 was originally discovered as a protein which was expressed in minute amounts together with Interleukin 8 (IL-8) by stimulated human osteosarcoma cells (Proost et al. (1993) Biochemistry 32, 10170-10177). The human gene for GCP-2 encodes a protein of 114 amino acids.
GCP-2, previously named ‘SCYB6’, and according to most recent terminology ‘CXCL6’, shows the strongest sequence similarity in coding and noncoding sequence to CXCL5 (SCYB5/ENA-78 (Epithelial cell-derived Neutrophil Attractant 78)).
In humans and cows the CXCL6 protein occurs in a number of N-terminally truncated forms which seem to have no different activity in a standard in vitro migration assay (Proost et al. (1993) cited above). Up to 28 N-terminally and/or C-terminally cleaved versions of murine CXCL6 were isolated from fibroblasts and epithelial cells (Wuyts et al. (1999) J. Immunol. 163, 6155-6163; Van Damme et al. (1997) J Leukoc Biol 6, 563-569). These N-terminally truncated versions of murine CXCL6 show dramatic differences in specific chemotactic potency towards both human and murine neutrophils in vitro and in vivo. CXCL6 is reported to be a specific granulocyte attractant having no chemotactic effect on monocytes (Van Damme et al. (1997) cited above).
As its name indicates CXCL6 is a CXC chemokine. It belongs to the subgroup of neutrophil activating chemokines acting through CXCR1 which are characterized by the N-terminally located Glu-Leu-Arg sequence (ELR motif). Together with IL-8, CXCL6 is the only ELR-containing chemokine that has a basic amino acid at the sixth position after the second cysteine of the CXC motif. Wolf et al. ((1998) Eur. J. immunol. 28, 164-170) have shown that this basic amino acid is an important determinant for CXCR1 activation. Wuyts et al. ((1997) cited above) further demonstrated that CXCL6 binds to both CXCR1 and CXCR2 receptors.
Chemokines in general have been connected to cartilage degradation in the context of the inflammatory reaction observed in rheumatoid arthritis. Borzi et al. ((1999) Febs Lett. 455, 235-242) and Pulsatelli et al. ((1999) J. Rheumatol. 26, 1992-2001) describe the expression of IL-8, Gro-alpha, MCP-1, RANTES, MIP-1alpha and MIP1beta in chondrocytes obtained from normal individuals, osteoarthritis (OA) and rheumatoid arthritis (RA) patients. Borzi et al. ((2002) Arthritis Rheum. 46, 3201-3211) suggest the existence of a novel catabolic pathway primed by chemokines and their receptors that leads to the breakdown of cartilage matrix components. The upregulation of chemokines, according to these authors, is related to the pathogenesis and persistence of the joint disease. Votta et al. ((2000) J Cell Physiol. 183, 196-207) suggest that the CC chemokine Ckbeta 8 plays a role in the recruitment of osteoclast precursors to sites of bone resorption. Osteoblasts on the other hand do not respond to this chemokine. Alaaeddine et al. ((2001) Arthritis Rheum. 44, 1633-1643) studied the expression of the chemokine RANTES (a member of the CC family) and its receptors in normal and OA cartilage and assigned a pathogenetic activity to this chemokine. Kanbe et al. ((2002) Arthritis Rheum. 46, 130-137) have suggested a role for the CXC chemokine SDF-1 in synovial cell mediated degradation of cartilage matrix in RA and OA. Wuyts et al. ((2003) Lab Invest. 83, 23-34) demonstrated that inflammatory cytokines such as IL-1 induce CXCL6 expression in chondrocytes, although this expression level is about 100 times less than IL-8.
Silvestri et al. ((2003) Rheumatology 42, 14-18) describe the expression of chemokine receptors, but not of the chemokines themselves, in inflammatory arthritis and osteoarthritis. It is postulated that the activity of chemokines tilts the balance of cartilage homeostasis towards degradation. EP 0804486 suggests the use of CXCL6 as a medicament for inflammatory conditions, while U.S. Pat. No. 6,410,268 indicates the possible use of GCP2-antagonists in the treatment of inflammatory diseases such as RA. The latter furthermore suggests the use of GCP-2 to stimulate woundhealing in the treatment of fibrotic diseases such as OA.
A role of chemokines in the migration of cell types which are unrelated to leukocytes has recently emerged. As cited above, a chemotactic effect of Ckbeta 8 on osteoclast precursors has been described (Votta et al. (2000) J Cell Physiol. 183, 196-207). Doitsidou et al. ((2003) Cell 11, 647-59) and Wright et al. ((2002) J Exp Med. 195, 1145-1154) demonstrate the role of SDF-1 and its receptor CXCR4 in the migration of primordial germ cells and hematopoietic stem cells respectively. King et al. ((2000) Blood 97, 1534-1542 and (2001) J. Immunol. 164, 3774-3782) describe a dramatic increase in hematopoietic activity upon amino-terminal truncation of CXCL2 (GroBeta) and demonstrate that this truncated version can mobilize hematopoietic stem cells. U.S. Pat. No. 6,410,268 speculates on a possible role in mobilisation of stem cells using CXCL6, in particular of bone marrow stem cells, which could be applied in the treatment of cancer and leukemia.